The invention relates to an ultrasonic liquid level sensor instrument that combines a continuous transmitter that transmits though the air for measuring the level of liquid in a vessel on a continuous basis and a point level sensor that sets a high level alarm.
Ultrasonic instruments for measuring the level of liquid in a vessel are well known. Included among the type currently used is the so-called air transmitter in which an ultrasonic transmitter/receiver transducer is located in a vessel at a certain height and transmits ultrasonic energy through the air to the liquid level surface. The time measured between the transmission and return of the ultrasonic energy is used to compute the distance from the transducer to the liquid level.
While such air type ultrasonic transmitter systems are quite serviceable, a problem arises where the level of the liquid in the vessel rises above the level of the transducer or the transducer fails. When either of such events occurs, the system has no way to determine the failure and there is no way to produce a signal to indicate such failure condition.
A problem of this type is found in an aircraft sanitary tank system having a receiving vessel which contains different kinds of wastes and liquid. Present systems in aircraft use a differential pressure transducer to continuously monitor the liquid level in the vessel. However, due to the dirty nature of the liquid, even when the vessel is not over-filled, the pressure sensing tube sometimes gets plugged. This causes false alarms to which service people must respond to service the tank when the aircraft is on the ground. Such false alarms are expensive to respond to in order to clean-up and repair. This presents a problem, particularly when maintenance must be provided on an expedited basis in order to maintain flight schedules, and increase maintenance costs.
The present invention relates to an ultrasonic instrument, which combines both an air type continuous level ultrasonic transmitter/receiver together with a gap type point level ultrasonic sensor. The continuous level transmitter/receiver provides a continuous measurement of the liquid level in the vessel and the point level sensor is used for sensing an overflow, or high level condition in the vessel. Each of the continuous transmitter/receiver and point level sections of the instrument has a self test capability.
The instrument of the invention includes a sensor fixture that holds both the continuous air transmitter/receiver transducer and the point level sensor. The fixture is mounted in a single hole in the wall of a tank or a vessel in which the liquid level is being monitored. In a preferred embodiment of the invention the continuous ultrasonic transducer transmitter/receiver is at the top of the fixture for transmitting ultrasonic energy through the air to the surface of the liquid during normal operation. The point level sensor of the fixture is formed by an air gap type ultrasonic transducer, preferably located below the continuous energy transducer, to sense an over-flow condition, i.e., when the liquid rises near to, but below, the level of the continuous air type transmitter/receiver transducer.
In accordance with the invention, the arrangement provides for both continuously measuring the distance from the instrument to the top of the liquid level i.e., to measure the level of the liquid in the tank, as well as an alarm system to sense when there is an overflow or failure of either of the continuous sensor or the point level sensor. Since the fixture uses a single hole for mounting as compared to existing three hole systems, that have one hole for the air transmitter and two for the point level sensor, the costs of mounting and the number of leak points in the sanitary tank are reduced.